Need a custom web-page where students upload files into a teacher's Dropbox. I use Dropbox for Business and created a web-page with upload files from my web-page to my dropbox folder with API/C#/OAuth2, what about other people uploading into my folder?

The API was designed with the intention that each user would link their own Dropbox account, in order to interact with their own files. However, it is technically possible to connect to just one account. The SDKs don't offer explicit support for it and we don't recommend doing so, for various technical and security reasons. Most of these concerns are allayed for server-side apps though.

However if you did want to go this route, instead of kicking off the authorization flow (i.e., sending the user to /authorize), you would manually use an existing access token for your account and app. (Just be careful not to revoke it, e.g. via https://www.dropbox.com/account/security .)

In your case, this means you would always set the token variable to a hard coded access token for your account in particular.